Murine L1210 cells in primary culture in common with a wide range of human tumor cells in primary culture require mercaptoethanol or other thiols for growth. We have observed that the copper-specific chelator, bathocuproine sulfonate, could also support the growth of L1210 cells in primary culture. By removing available copper ions normally present in the tissue culture medium, it retarded the oxidation of cysteine to poorly utilized cystine. Evidence that copper ion was inhibitory only by functioning as an extracellular catalyst for oxidation of cysteine was supported by the observation that the mixed disulfide of methyl mercaptan and cysteine, 4-thiamethionine, supported good growth of the cells in primary culture even in the presence of 100 MuM copper sulfate. 4-Thiamethionine is transported into the cells by the same mechanism as is methionine but is reduced to methyl mercaptan and cysteine inside the cell. It therefore permits the cell to grow where cystine transport is ineffective and where cysteine is rapidly oxidized to the non-utilized cystine.